Identify my new invader (Potentially a flatworm)

[Kernam]

Hello all!

I've never had an issue that has stumped me as much as these little guys have! So I decided it was high time I came down off my mountain of solitude and ask you fine folks for some help.

So here's the skinny. I noticed these individuals, when I was doing a water change yesterday, in the return section of my sump. Naturally, I combed the rest of the tank to see if they were in other locations or not. I found around ten to fifteen in the overflow of the tank, five in the bio balls that are visible in the sump and nine in the sump return section. The larger ones are around the length of a grain of rice. I took a quick look at one under a random scope I have at home with a 10/0.25 magnification (was an old scope from work they were going to toss because the other objectives are busted) the body and mouth(?) are covered in hair-like filaments.

Thought they looked odd but I was fairly certain I would be able to ID them through some research throughout the evening. My overconfidence hard at work! Naturally, that did not happen. So at work today I strolled through the lab asking some of the aquaculturists, fellow staff biologists and my PhDs what terrifying horror had found its way into my Cetropyge tank. My answers ranged from a species of hydroid to flat worms, but no one seemed confident in their identification.

So I ask you internet community, what do you all think these guys are?

<a href="https://www.flickr.com/photos/128237622@N05/15646640215" title="DSC01742 by Ron Hans, on Flickr"><img src="https://farm4.staticflickr.com/3944/15646640215_d7c7bca845_c.jpg" width="800" height="600" alt="DSC01742"></a>
<a href="https://www.flickr.com/photos/128237622@N05/15026473443" title="DSC01721 by Ron Hans, on Flickr"><img src="https://farm6.staticflickr.com/5615/15026473443_32c1a54672_c.jpg" width="800" height="600" alt="DSC01721"></a>
<a href="https://www.flickr.com/photos/128237622@N05/15647460072" title="DSC01748 by Ron Hans, on Flickr"><img src="https://farm6.staticflickr.com/5601/15647460072_7520cb8878_c.jpg" width="800" height="600" alt="DSC01748"></a>

 

[Adetia]

Pineapple sponges; they are harmless.

 

[Betta132]

Yep. All they do is sit and filter feed... not dangerous. They don't sting, either, they're soft and mushy.
Nobody thought to ask if they moved?

 

[Kernam]

It came up but to be fair, my lab deals almost exclusively with pelagic fish for wild stock enhancement and food production. I'm one of the few hobby aquarists, as well. So while the lab has a wealth of information on fish at its disposal, many components of the reef system (inverts, coral, etc.) are not things that my co-workers have much insight or interest in. Many times I, ironically, even end up being the odd ball out in conversations because I tend to steer the conversation into the realm of reef talk

Anyway, thank you for the quick identification! As my first experience on the site, it was painless and informative. I'll try to make it a point to pop in regularly!